WiFi, also referred to as wireless local area network (WLAN), uses IEEE 802.11 standard technologies as an air interface, including physical (PHY) and media access control (MAC) layers. In an IEEE 802.11 WLAN, the communication channel is shared by multiple stations, which may be coordinated by a distributed channel access function, i.e., a distributed coordination function (DM, based on the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism.
The DCF uses both physical and virtual carrier sense functions to determine the state of the medium. The physical carrier sense function resides in the PHY layer and uses energy detection and preamble detection to determine whether the medium is busy. The virtual carrier sense function resides in the MAC layer and uses reservation information carried in the duration field of the MAC header, which indicates the impending use of the wireless channel. The virtual carrier sense mechanism is called the network allocation vector (NAV). The wireless channel is determined to be idle only when both the physical and virtual carrier sense mechanisms indicate it to be so.
A station (STA) with a data frame for transmission may perform a clear channel assessment (CCA) by sensing the wireless channel for a fixed duration, i.e., the DCF inter-frame space (DIFS). If the wireless channel is busy, the station waits until the channel becomes idle for a DIFS, and then waits for a further random backoff period. The backoff timer decreases by one for every idle slot and freezes when the channel is sensed busy. When the backoff timer reaches zero, the station starts data transmission. In WiFi networks, the downlink traffic from an access point to a station is announced via a traffic indicator map (TIM).